1. Field of the Invention
The present invention relates to a structure of a wire dot-printing head.
2. Description of the Prior Art
A wire dot-printing head is known of such a type that an elastic printing hammer is attracted by the magnetic force of a permanent magnet in non-printing operation while it is, in printing operation, released by cancelling out the magnetic force. This is disclosed, for example, in U.S. Pat. No. 4,225,250 (SEGMENTED-RING MAGNET PRINT HEAD). Japanese Utility Model Laid-Open Publication No. 58-151052 also discloses this wire dot-printing head. The latter will be described with reference to the accompanying drawings.
Referring to FIGS. 1 and 2, each showing a portion of a printing head, with a coil 7 not conducted, magnetic flux of a permanent magnet 1 passes successfully through a core 2, an armature 3, an armature yoke 4, a spacer yoke 5 and a yoke 6. The core 2 and the armature 3 are magnetically attracted to each other, whereby a leaf spring 8 is deflected and deformed.
With the coil 7 conducting current in this state, magnetic force from the coil 7 cancels out the magnetic force of the permanent magnet 1 in a gap between the armature 3 and the core 2, and thereby the armature 3 is released from the core 2. Hereupon, deformation energy stored in the leaf spring 8 is released, and the armature 3 fixedly mounted on the end of the leaf spring 8 is rotated clockwise whereby a printing wire fixed on the armature jumps out.
Mounting error upon mounting the leaf spring 8 on the armature 3 is inevitable. Thereupon, deflection of the leaf spring 8 is scattered within the printing head due to the error, whereby impact force of each printing wire is also scattered and prevents high quality and high speed printing to be effected. The auxiliary leaf spring 10 and the adjusting screw 11 serve to adjust a deflection of the leaf spring 8 to reduce the scatter of the impact force.
In addition, an auxiliary coil spring 12 and an adjusting screw 13 shown in FIG. 2 serve the same function as the auxiliary leaf spring 10 and the adjusting screw 11 shown in FIG. 1, respectively.
However, this prior printing head, even if adjusted, suffers from problems as follows.
(1) Even if the printing head is adjusted by the auxiliary leaf spring 10 and the adjusting screw 11 as shown in FIG. 1, the armature 3, as it rotates on a corner 2a of the core 2, slides in contact with a contact part 3a of the leaf spring 10. Therefore, with increased friction therebetween, the operating characteristics become unstable, while with increased wear therebetween the impact force is reduced and thereby the service life of the printing head is shortened. PA0 (2) Even if the printing head is adjusted by the auxiliary coil spring 12 and the adjusting screw 13 as shown in FIG. 2, the operating characteristics also become unstable as in the case of the prior printing head shown in FIG. 1, and thereby the service life is likewise deteriorated.